once: make helper generic for calling functions once
Make the get_random_once() helper generic enough, so that functions
in general would only be called once, where one user of this is then
net_get_random_once().
The only implementation specific call is to get_random_bytes(), all
the rest of this *_once() facility would be duplicated among different
subsystems otherwise. The new DO_ONCE() helper will be used by prandom()
later on, but might also be useful for other scenarios/subsystems as
well where a one-time initialization in often-called, possibly fast
path code could occur.
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/once.h b/include/linux/once.h
index 2a83b53..285f12c 100644
--- a/include/linux/once.h
+++ b/include/linux/once.h
@@ -4,21 +4,54 @@
#include <linux/types.h>
#include <linux/jump_label.h>
-bool __get_random_once(void *buf, int nbytes, bool *done,
- struct static_key *once_key);
+bool __do_once_start(bool *done, unsigned long *flags);
+void __do_once_done(bool *done, struct static_key *once_key,
+ unsigned long *flags);
-#define get_random_once(buf, nbytes) \
- ({ \
- bool ___ret = false; \
- static bool ___done = false; \
- static struct static_key ___once_key = \
- STATIC_KEY_INIT_TRUE; \
- if (static_key_true(&___once_key)) \
- ___ret = __get_random_once((buf), \
- (nbytes), \
- &___done, \
- &___once_key); \
- ___ret; \
+/* Call a function exactly once. The idea of DO_ONCE() is to perform
+ * a function call such as initialization of random seeds, etc, only
+ * once, where DO_ONCE() can live in the fast-path. After @func has
+ * been called with the passed arguments, the static key will patch
+ * out the condition into a nop. DO_ONCE() guarantees type safety of
+ * arguments!
+ *
+ * Not that the following is not equivalent ...
+ *
+ * DO_ONCE(func, arg);
+ * DO_ONCE(func, arg);
+ *
+ * ... to this version:
+ *
+ * void foo(void)
+ * {
+ * DO_ONCE(func, arg);
+ * }
+ *
+ * foo();
+ * foo();
+ *
+ * In case the one-time invocation could be triggered from multiple
+ * places, then a common helper function must be defined, so that only
+ * a single static key will be placed there!
+ */
+#define DO_ONCE(func, ...) \
+ ({ \
+ bool ___ret = false; \
+ static bool ___done = false; \
+ static struct static_key ___once_key = STATIC_KEY_INIT_TRUE; \
+ if (static_key_true(&___once_key)) { \
+ unsigned long ___flags; \
+ ___ret = __do_once_start(&___done, &___flags); \
+ if (unlikely(___ret)) { \
+ func(__VA_ARGS__); \
+ __do_once_done(&___done, &___once_key, \
+ &___flags); \
+ } \
+ } \
+ ___ret; \
})
+#define get_random_once(buf, nbytes) \
+ DO_ONCE(get_random_bytes, (buf), (nbytes))
+
#endif /* _LINUX_ONCE_H */